Scanning method for storage tubes



May 13, 1941;

U. KNICK SCANNING METHOD FOR STORAGE TUBES Filed March 4, 1938 PatentedMay 13, 1941 UNHTE ST g 5 i i i FFEE SiJ'ANNING METHOD FOR STORAQE.TUBES Application March 4, 1938, Serial No. 193,987 In Germany March 6,1937 2 Claims.

In known image analyzing tubes possessing a storage member, a certainmargin around the scanned picture area is always present which is notscanned by the electron beam, nor illuminated by the light image.Indeed, in production of the storage member, which usually consists of aphotomosaic deposited on an insulating coating over a conductive plateas is Well known in the art, it is necessary-or at least preferable-toleave a margin around the mosaic area. Furthermore, a margin which isnot scanned has been required because the scanning beam could,otherwise, be deflected over the edge of thestorage member into space,whereby large interfering voltages are generated, as has beenexperienced.

This margin, however, when not scanned, causes the disadvantage thatthose peripheral portions of the storage member which are not scannedbecome increasingly negatively charged by stray electrons. This highnegative charge destroys the uniformity of the accelerating fieldgenerated in front of the storage member, and as a consequence thereofcauses a non-uniform distribution of secondary electrons liberated fromthe edges of the scanned portion of the storage member by impact of thescanning beam. In practical use this non-uniform field condition causesbright edges in the picture field at the receiver which are also presentwhen the mosaic is not illuminated.

A metallic coating of the margin outside the scanned area, in order thata fixed potential may be applied to it, is out of question becausegreater capacitance between signal plate and ground would be obtained.Another reason for the inadvisability of doing this is that a greatnumber of secondary electrons can be produced if the scanning beamimpacts this metallic coating, when slight disturbances occur in thedeflection, and these secondary electrons would destroy the chargedistribution on the adjacent portions of the mosaic.

It is the object of the present invention to bring the margins, at leastpartially, to a defined potential by means of the cathode ray beam. Theelectron beam is, therefore, deflected over a margin of, for instance,one to two centimeters in width, in addition to the picture area proper.The margin then acquires the same potential throughout, approximatelyminus 5 volts, with respect to the accelerating electrode, and has thestorage condition of no light, because obviously no light image is to bethrown on the margin. The scansion of the margin must, obviously, notsubtract from the total time available for scansion of the picture area.Consequently, the scanning beam must pass over the margins at a higherrate of speed than ordinarily and the retrace period must also beshorter than is common in the normal process.

The invention is of particular importance to the vertical margins, whichcan readily be brought to a defined potential by a corresponding waveshape of the line deflecting fields.

In the attached drawing, the solid curve a shows an example of the linedeflecting voltage curve according to the method of the invention, thiswave form being readily obtainable by those skilled in the art, whilethe dotted curve b shows the voltage wave used for normal scansion. Anysignal voltages generated during scansion of the margin should besuppressed, or compensated for, by suitable measures. It may also bearranged that they occur at such a time that they coincide with thesynchronizing interval. The synchronizing impulses are generated in theusual manner by means of a separate circuit.

I claim:

1. The method ofoperating an image analyzing tube of the type having astorage surface which comprises the steps of scanning substantially thewhole area of said storage surface, projecting the image on only thecentral portion of said storage surface, maintaining the speed ofscanning substantially constant over said central portion, andincreasing the speed of scanning over the remaining portion of saidstorage surface.

2. The method of operating an image analyzing tube of the type having astorage surface which comprises the steps of scanning substantially thewhole area of said storage surface, projecting the image on only thecentral portion of said storage surface, maintaining the speed ofscanning substantially constant over said central portion, increasingthe speed of scanning over the remaining portion of said storagesurface, and reducing the retrace time by an amount equal to the time ofscanning the remaining portion of said storage surface.

ULRICH KNICK,

